The present invention relates to a method and a system for initiating a communication, particularly a communication link. Specifically, the present invention relates to the initiating of closed-loop power control and to a method and a system for controlling power.
In mobile communication systems, it is known to start the initiation of a communication by sending an initiation signal from a mobile station such as, for example, a mobile telephone (also called user equipment (UE)) to a base station (also called node B). The initiation signal can be a so-called RACH (random access channel) message or a preamble; i.e., a specific message preceding a possible RACH message.
A particularly important element of CDMA (code division multiple access) systems such as, for example, the UMTS (Universal Mobile Telecommunications System) is formed by the closed-loop power control. The closed-loop power control method used for the transmission of data here cannot be used for the initiating RACH transmission since it cannot be performed before the transmission begins. For this reason, a so-called open-loop power control method is used for the initiating RACH transmission. Since it is not possible with this method to determine the ideal uplink power (power of a signal which is sent from the mobile station to the base station and arrives at the base station with the power required for reception) with high accuracy, a so-called power ramping method is additionally used in which RACH messages are repeatedly transmitted by the mobile station with increasing power. If the power of the transmitted RACH message is too low, the message is not received by the base station and no acknowledgement message is transmitted from the base station to the mobile station and the transmission of the RACH message from the mobile station to the base station is repeated with higher power. When the power of the RACH message is high enough, the message is received by the base station and an acknowledgement message is transmitted from the base station to the mobile station. Instead of a complete RACH message, it is also possible to send only a shortened signal, a so-called preamble.
Such a method is known, for example, from EP 0917304 A2. In this method, the transmission of an initiating signal is repeated with increasing power, starting from a determined initial power, until either an acknowledgement message is received or the number of repetitions exceeds a threshold value. When an acknowledgement message is received, the transmission of data is continued. If the number of repetitions exceeds a threshold value, the method is begun again with the transmission of an initiating signal with an initial power.
A method of this type which has been developed further is known from “ETSI TS 125.214, V3.4.0-DRAFT (2000-09), UMTS, Physical layer procedures (FDD), (Release 1999)”. In this case, the so-called random access procedure is described, in particular in Chapter 6.1, which procedure generally describes the initiation of the communication and, in particular in steps 4 to 7 and 9 to 10, describes the initiation of the closed-loop power control. The initiating message which is sent from the mobile station to the base station is called a preamble in this case. In the text which follows, this method will be explained briefly and, for the purpose of simplification, the steps which are not required for understanding the closed-loop power control method will not be mentioned; in particular, the allocation of various time slots or channels to various mobile stations will not be discussed in greater detail.
In step 4 of the method described in 6.1, a counter for counting the retransmissions of the preamble is set to the value Preamble Retrans Max.
In step 5, the power for the transmission of the preamble is set to the value Preamble Initial Power.
In step 6, a preamble with the “preamble transmission power” power is sent in an access time slot from the mobile station to the base station.
If, in step 7, the mobile station does not detect any acknowledgement message (neither a positive acknowledgement message (“positive acquisition indicator”) nor a negative acknowledgement message (“negative acquisition indicator”)), the power for transmitting the preamble is increased by ΔP0=Power Ramp Step [dB] in step 7.3.
In step 7.4, the counter for counting the retransmissions of the preamble is reduced by one.
If, in step 7.5, the counter for counting the retransmissions of the preamble is >0, the method continues at step 6 and, if not, it is found that no acknowledgement message has been received and the procedure is exited.
If a negative acknowledgement message is received, the procedure is exited in step 8.
In step 9, a random access message is transmitted to the base station in the case where a positive acknowledgement message (“positive acquisition indicator”) has been received.
In step 10 it is found that the random access message has been transmitted.
In practice, however, the power with which a mobile station is allowed to transmit within a mobile radio system is limited by a maximum power and, when this is exceeded, it can lead to an impairment in the quality of transmission; for example, due to deteriorated phase accuracy or modulation accuracy. In addition, the power can be limited differently from cell to cell in order to reduce interference. Furthermore, the maximum power of a mobile station can be limited by rules of regulation or by a standard to be applied. In addition, mobile stations are not capable of transmitting at an arbitrarily low power. For this reason, minimum powers with which a mobile station must be able to transmit are defined in mobile radio standards.
The present invention is thus directed to a method and a system for initiating a communication, particularly a method and a system for controlling power via which the restrictions with regard to the transmitting power of mobile stations, given in practice, are taken into account in such a manner that, compared with the prior art, less energy is consumed and less interference is caused in the mobile radio system.